1. Field of the Invention
The present invention relates to an air-fuel ratio control device for an internal combustion engine.
2. Description of the Related Art
In a general air-fuel ratio feedback control, an air-fuel ratio feedback correction amount FAF is calculated in accordance with an output of an air-fuel detecting sensor (hereinafter referred to as an O.sub.2 sensor (oxygen concentration detection sensor)), and the air-fuel ratio is controlled in accordance with FAF. In this air-fuel ratio feedback control, when the fuel supply system is in an abnormal state, FAF is stuck at a lower guard or an upper guard thereof. For example, when a fuel injection amount is too little due to the abnormal state of the fuel supply system, the output of the O.sub.2 sensor remains lean, and thus FAF is stuck to the upper guard. Conversely, when the fuel injection amount is too large, the output of the O.sub.2 sensor remains rich, and thus FAF is stuck to the lower guard. Accordingly, when FAF is stuck to the upper guard or the lower guard, it can be determined that the fuel supply system is in an abnormal state.
In the air-fuel ratio feedback control into which a learning correction amount is introduced, however, a fluctuation of an average FAFAV of FAF is absorbed by the learning correction amount FG. Accordingly, if it is determined that the fuel supply system is in the abnormal state by detecting that FAF is stuck to the upper or lower guard, the determination of the abnormal state is delayed. Therefore, when a sum of FAF and FG (FAF+FG) or a sum of FAFAV and FG (FAFAV+ FG), or a product of FAF and FG (FAF.multidot.FG) or a product of FAFAV and FG (FAFAV.multidot.FG) is very different from a predetermined value, it is determined that the fuel supply system is in the abnormal state (see TOYOTA Technical Publication 1389).
Note, in TOYOTA Technical Publication 1389, an engine having first and second cylinder banks is disclosed. In this engine, it is determined whether or not the fuel supply system is in the abnormal state, on the basis of a difference between a sum (or a product) of FAF1 and FG1 of the first cylinder bank and a sum (or a product) of FAF2 and FG2.
Nevertheless, when the learning correction amount FG is renewed, FAF is gradually changed to become an objective value, by an integration amount corresponding to an amount of change of the learning correction amount FG.
For example, referring to FIG. 13, when the learning correction amount FG1 of the first (right) cylinder bank is increased at t.sub.1, the air-fuel ratio feedback correction amount FAF1 of the right cylinder bank is gradually decreased by the integration amount, to compensate the increase of FG1, but the average FAFAV1 of air-fuel ratio feedback correction amount FAF1 of the right cylinder bank is not decreased until t.sub.2, at which FAF1 is inverted, since the calculation timing of FAFAV1 is when FAF1 is reversed. As a result, between t.sub.1 and t.sub.2, since (FAFAV1+FG1) becomes too large, .vertline.(FAFAV1+FG1)-(FAFAV2+FG2).vertline. becomes an abnormal value. Therefore, a problem arises in that it is misdetermined that the fuel supply system is in the abnormal state.
Note that this problem occurs even if FAFAV1 is calculated at certain time periods, because FG is changed by a relatively large amount when it is renewed, but FAF is changed by a relatively small amount, which is the integration amount, when it is renewed. In other words, because it takes a certain time for FAF to become the value that indicates the necessary correction amount to make the air-fuel ratio close to the target air-fuel ratio with the renewed FG, the problem arises.